The invention relates to motors for electric power tools. More particularly, the invention relates to a braking and control circuit for such a motor.
AC universal motors have commonly been used in electric power tools, such as drills, circular saws and other types of equipment. Generally, such motors provide a high-power, light-weight power source for these power tools. Typically, the universal motor includes a housing, a stator assembly having a run coil, and a rotatable shaft or arbor having an armature mounted thereon. Current flows through the run coil and creates a magnetic field which interacts with the magnetic field of the armature. This interaction rotatably drives the arbor. To drive a tool element, such as a drill bit or a saw blade, the tool element may be mounted directly on the arbor or be coupled to the arbor by a gear transmission or the like.
Conventional universal motors tend to coast, i.e., the arbor continues to rotate for some time after the motor is disconnected from the electrical power source. This coasting generally results from the rotational momentum of the arbor, the transmission, and the tool attachments. To prevent or limit coasting, the motor often includes a braking arrangement.
A typical braking arrangement includes a dynamic braking circuit which relies on passive generation of free wheeling current in the stator to produce a counter-electromagnetic force (counter-EMF) to stop the rotation of the arbor and to, thereby, brake the motor. One such dynamic braking circuit is shown and described in U.S. Pat. No. 5,294,874.